After ammunition, the second most significant usage of explosives in the community is for blasting. This activity, must only be carried out by licensed operators.

When explosives are packaged for transport and storage, a distinctive diamond- shaped label must appear on the package. This label has an orange background, with the word ‘Explosive’ in black letters in the centre. A black exploding object appears at the top of the label or this may be replaced with a number (e.g. 1.4).

An explosive is defined as ‘a material which, when suitably initiated, decomposes with the rapid formation of a large volume of gas at high temperature’. Explosives may be solid, liquid or gaseous, and may be single substances or mixtures of different substances.

In most explosives (but not all), the process by which large quantities of heat and gas are produced is oxidation, i.e. combustion in which the oxidiser is provided by the explosive, unlike normal combustion where the oxidiser is atmospheric oxygen.

Once this oxidation process in the explosive has been initiated, it will proceed without any additional energy or material being required, until the explosive has been fully consumed.

In some explosives, such as gunpowder, the fuels (carbon and sulphur), are present as separate ingredients mixed intimately with the oxidiser (potassium nitrate). In other explosives the fuel and oxidiser, are present within the same molecule e.g. trinitrotoluene (TNT), where the fuels are carbon and hydrogen atoms, and the oxidisers are nitro (NO2) groups.

It is convenient to divide explosives into different groups, characterised largely by their different rates of combustion. The precise rate of combustion for each explosive, is dependent on the chemical composition, the physical state, the degree of confinement and the means used to initiate the combustion. Explosives can be regarded simply as a source of chemical energy.

The main groups, in order of decreasing rate of combustion are:

HIGH EXPLOSIVES

These are defined as explosives, capable of detonation, and by which by production of blast and/or fragments of their container, are used for disruptive purposes.

Detonation , is the passage of an exothermic reaction wave through the explosive , following and supporting the shock front. The velocity of this wave is typically in the range, of 3000 to 10000 m/s.

The rate at which energy is released, is the main feature distinguishing high explosives from propellants. It is from this feature that the terminology ‘high’ or ‘low’ explosives, is derived.

Within these two categories there are subdivisions, based on other characteristic properties.

One such subdivision of high explosives is that of primary or secondary explosives. This classification essentially relates to the position of the explosive, in the explosive train used to control the initiation of a munition or charge.

A ‘primary explosive’ is a sensitive explosive, which is readily ignited and exploded. Primaries are used to initiate other explosives, and are often referred to as initiators. Some examples are : lead azide, lead styphnate, tetrazine.

An ‘intermediate explosive’ is used to augment the initiatory impulse, in order to cause detonation of the main explosive charge. These are really secondary explosives used in an intermediary role. Some examples are : PETN, RDX/wax.

A ‘secondary explosive’ is a substance or mixture, which will detonate when initiated by a shock wave, however it will not normally detonate, when heated or ignited. Some examples are : TNT, RDX/TNT.

PROPELLANTS

These are explosives, which can burn in a rapid but controlled manner, i.e. without detonating, even when strongly confined e.g. within a gun barrel.

The pressure of the combustion gases, is used to drive a bullet or shell along the bore of a gun, or to propel a rocket.

The rate of burning is dependent on pressure, and is generally between 1 and 250 m/s.

Propellants can be either solid or liquid. In the space shuttle, both are used to propel the craft into space. The shuttle’s main engines burn liquid hydrogen and liquid oxygen, the main booster rockets use a solid propellant with ammonium perchlorate as oxidiser.

In most conventional ammunition, the propellant is solid. The main type used is based on nitrocellulose. Nitrocellulose is prepared by nitration of cellulose from trees or cotton, with nitric acid.

Raw nitrocellulose is sensitive to friction. In the solvent manufacturing process, the propellant usually contains residual solvent (typically diethyl ether), which increases the propellant’s sensitivity to static electricity. Solventless propellants have been developed, however they are not widely used.

During manufacture, propellants are extruded through a die, and then cut to length. The length and diameter are varied to suit the final use. For example, propellant used in small arms ammunition, has a small diameter and length, whereas propellant used in large caliber ammunition, has larger grain diameter and length.

When propellant is ignited, it burns quite evenly from all surfaces. If the propellant grain is a cylinder, its burning surface decreases as it burns, resulting in decreased gas production. This is undesirable, and is corrected by perforating the propellant grains, when the propellant is extruded through the die.

The resulting holes provide surfaces, which increase as the propellant burns, and compensates for the decreasing outer surface.

Some propellants absorb moisture, which can cause instability during storage, and cause ‘muzzle-flash. All service propellants are stabilised by the addition of chemical additives.

Propellants based on nitrocellulose alone are called ‘single-base’ propellants. There are applications, where a higher power is needed. This requirement is met by ‘double-base’ propellants in which an explosive (nitroglycerine) is added to the nitrocellulose, during manufacture.

PYROTECHNICS

These are explosives, which burn at slower rates than propellants, and are used as sources of intense light (coloured and white), noise and as igniters to priming and delay compositions.

The rate of burning is again dependent on pressure, and is generally between 0.1 and 15 m/s.

Pyrotechnic compositions are generally composed of an intimate mixture of finely divided combustible substances, and oxidants which upon ignition, can either burn to produce light or smoke, or explode to produce noise.

The purpose of an oxidant is to provide oxygen for the combustion of the fuel. Some examples of oxidants are: metal nitrates, ammonium nitrate, potassium chlorate, potassium perchlorate.

The purpose of a fuel is to generate energy during combustion. Fuels can be either metal powders, such as magnesium (used in flares), or carbon based, such as lactose (used in smoke generation).

Colour can be produced by the presence of metal nitrates in the pyrotechnic composition. These are used in signal flares, tracers or in fireworks, and can produce colours such as green, blue, yellow, orange, purple and red.

An example of a pyrotechnic is the common safety match. The striker pad on the matchbox consists of a composition of red phosphorus, which is sensitive to friction.

There are four main types of ignition mechanisms. They are:

• Friction
• Impact
• Sparks
• Heat

The likelihood of an explosive igniting is a function of the sensitivity.

The more sensitive, the more likely it is, that ignition will occur. The sensitivity can be found by:

• Experience (not a good idea)
• Guessing (definitely not a good idea)
• Careful experiments (recorded as Explosive Safety Certificates).

Explosive Safety Certificates typically report : ‘sensitiveness to direct mechanical shock’, ‘sensitiveness to friction’, temperature of ignition’, ‘inflammability’, ‘behaviour on inflammation’, ‘chemical stability’, ‘special precautions’, ‘poisonous ingredients’, ‘method of preparing and filling’, and ‘general precautions to be observed during manufacture and use’.

Once ignited and explosive can burn, at a controlled rate (when heat and gas are free to escape). It can deflagrate, when the burning rate increases ( due to thermal conduction and radiation). It can detonate, when the deflagration front reaches shock wave conditions (heat and pressure reinforce the shock front, temperatures of 3500 degrees Celcius and pressures of 300000 atmospheres, are common).

Wood has a higher energy content than most explosives, however it’s energy of activation is much greater, and its energy is released over a much longer period. Propellants have a low activation energy, and release their energy over a much shorter time.

Alan Cotterell

Acotrel Risk Management Pty Ltd

16^th August 1999